As an imaging method that uses an X-ray CT (Computed Tomography) apparatus, there is a dual energy imaging method (Patent Document 1). The dual energy imaging method is a technique utilizing a property (energy dependence) that a mass attenuation coefficient varies depending on a material and X-ray energy, and an image of an identical subject is taken using two different types of tube voltage, i.e., different energy distributions, thereby obtaining information regarding a material composition of the subject.